JUST WATCHED

Sciutto: Isis planning attacks on the US

MUST WATCH

Story highlights

European militants who have traveled to Syria could go back home and launch attacks

So far, militants in Iraq and Syria haven't seen attacking the West as a priority

But officials also fear the conflict could create new terror ties, bomb technology, lone wolves

The U.S. air strikes carried out Friday against ISIS are likely to have one unintended consequence: immediately increase the terrorist threat in Europe and the United States, Western counter-terrorism officials tell CNN.

Within hours of U.S. military jets and drones conducting a strike on ISIS artillery that had been used against Kurdish forces defending Irbil, ISIS supporters called for retaliatory attacks against the United States.

"It is a clear message that the war is against Islam and the mujahideen. The mujahideen must strive and seek to execute proactive operations in their own home, America, to discipline America and its criminal soldiers," Abu al-Ayna al-Khorasani, an administrator of Shumukh al-Islam, the top-tier forum for ISIS propaganda, wrote on his account Friday, according to a translation by the SITE Intelligence group.

Other ISIS supporters railed against the United States using the Twitter handle #AmessagefromISIStoUS, posting images of the wreckage of the twin towers. "Don't forget 11 Sept .. Maybe US citizens want more like that," one extremist tweeted. In June after ISIS captured Mosul, its supporters had warned against strikes in a Twitter campaign #CalamityWillBefallUS.

Here are three ways ISIS and its supporters could retaliate for U.S. military action against its fighters:

JUST WATCHED

Stopping the next domestic terror attack

MUST WATCH

JUST WATCHED

Refugees risk all to escape ISIS

MUST WATCH

Refugees risk all to escape ISIS02:42

Photos:The ISIS terror threat

Photos:The ISIS terror threat

Wounded passengers are treated following a suicide bombing at the Brussels Airport on March 22, 2016. The attacks on the airport and a subway killed 32 people and wounded more than 300. ISIS claims its "fighters" launched the attacks in the Belgian capital.

Hide Caption

1 of 68

Photos:The ISIS terror threat

Syrians gather at the site of a double car bomb attack in the Al-Zahraa neighborhood of the Homs, Syria, on February 21, 2016. Multiple attacks in Homs and southern Damascus kill at least 122 and injure scores, according to the state-run SANA news agency. ISIS claimed responsibility.

Hide Caption

2 of 68

Photos:The ISIS terror threat

Syrian pro-government forces gather at the site of a deadly triple bombing Sunday, January 31, in the Damascus suburb of Sayeda Zeynab. ISIS claimed responsibility for the attack, according to a statement circulating online from supporters of the terrorist group.

Wounded people are helped outside the Bataclan concert hall in Paris following a series of coordinated attacks in the city on Friday, November 13. The militant group ISIS claimed responsibility for the attacks, which killed at least 130 people and wounded hundreds more.

Hide Caption

6 of 68

Photos:The ISIS terror threat

Emergency personnel and civilians gather at the site of a twin suicide bombing in Beirut, Lebanon, on Thursday, November 12. The bombings killed at least 43 people and wounded more than 200 more. ISIS appeared to claim responsibility in a statement posted on social media.

Hide Caption

7 of 68

Photos:The ISIS terror threat

Smoke rises over the northern Iraqi town of Sinjar on November 12. Kurdish Iraqi fighters, backed by a U.S.-led air campaign, retook the strategic town, which ISIS militants overran last year. ISIS wants to create an Islamic state across Sunni areas of Iraq and Syria.

Hide Caption

8 of 68

Photos:The ISIS terror threat

Syrian government troops walk inside the Kweiras air base on Wednesday, November 11, after they broke a siege imposed by ISIS militants.

Hide Caption

9 of 68

Photos:The ISIS terror threat

Members of the Egyptian military approach the wreckage of a Russian passenger plane Sunday, November 1, in Hassana, Egypt. The plane crashed the day before, killing all 224 people on board. ISIS claimed responsibility for downing the plane, but the group's claim wasn't immediately verified.

Hide Caption

10 of 68

Photos:The ISIS terror threat

An explosion rocks Kobani, Syria, during a reported car bomb attack by ISIS militants on Tuesday, October 20.

Hide Caption

11 of 68

Photos:The ISIS terror threat

Shiite fighters, fighting alongside Iraqi government forces, fire a rocket at ISIS militants as they advance toward the center of Baiji, Iraq, on Monday, October 19.

Hide Caption

12 of 68

Photos:The ISIS terror threat

Smoke rises above a damaged building in Ramadi, Iraq, following a coalition airstrike against ISIS positions on Saturday, August 15.

Hide Caption

13 of 68

Photos:The ISIS terror threat

Iraqi men look at damage following a bomb explosion that targeted a vegetable market in Baghdad on Thursday, August 13. ISIS claimed responsibility for the attack.

Hide Caption

14 of 68

Photos:The ISIS terror threat

In this image taken from social media, an ISIS fighter holds the group's flag after the militant group overran the Syrian town of al-Qaryatayn on Thursday, August 6, the London-based Syrian Observatory for Human Rights reported. ISIS uses modern tools such as social media to promote reactionary politics and religious fundamentalism. Fighters are destroying holy sites and valuable antiquities even as their leaders propagate a return to the early days of Islam.

Hide Caption

15 of 68

Photos:The ISIS terror threat

An ISIS fighter poses with spoils purportedly taken after capturing the Syrian town of al-Qaryatayn.

Hide Caption

16 of 68

Photos:The ISIS terror threat

Smoke rises as Iraqi security forces bomb ISIS positions in the eastern suburbs of Ramadi, Iraq, on August 6.

Hide Caption

17 of 68

Photos:The ISIS terror threat

Buildings reduced to piles of debris can be seen in the eastern suburbs of Ramadi on August 6.

Hide Caption

18 of 68

Photos:The ISIS terror threat

The governor of the Asir region in Saudi Arabia, Prince Faisal bin Khaled bin Abdulaziz, left, visits a man who was wounded in a suicide bombing attack on a mosque in Abha, Saudi Arabia, on August 6. ISIS claimed responsibility for the explosion, which killed at least 13 people and injured nine others.

Hide Caption

19 of 68

Photos:The ISIS terror threat

Saudi officials and investigators check the inside of the mosque on August 6.

Hide Caption

20 of 68

Photos:The ISIS terror threat

Mourners in Gaziantep, Turkey, grieve over a coffin Tuesday, July 21, during a funeral ceremony for the victims of a suspected ISIS suicide bomb attack. That bombing killed at least 31 people in Suruc, a Turkish town that borders Syria. Turkish authorities blamed ISIS for the attack.

Hide Caption

21 of 68

Photos:The ISIS terror threat

Protesters in Istanbul carry anti-ISIS banners and flags to show support for victims of the Suruc suicide blast during a demonstration on Monday, July 20.

Hide Caption

22 of 68

Photos:The ISIS terror threat

People in Ashmoun, Egypt, carry the coffin for 1st Lt. Mohammed Ashraf, who was killed when the ISIS militant group attacked Egyptian military checkpoints on Wednesday, July 1. At least 17 soldiers were reportedly killed, and 30 were injured.

Hide Caption

23 of 68

Photos:The ISIS terror threat

Syrians wait near the Turkish border during clashes between ISIS and Kurdish armed groups in Kobani, Syria, on Thursday, June 25. The photo was taken in Sanliurfa, Turkey. ISIS militants disguised as Kurdish security forces infiltrated Kobani on Thursday and killed "many civilians," said a spokesman for the Kurds in Kobani.

Hide Caption

24 of 68

Photos:The ISIS terror threat

Residents examine a damaged mosque after an Iraqi Air Force bombing in the ISIS-seized city of Falluja, Iraq, on Sunday, May 31. At least six were killed and nine others wounded during the bombing.

Hide Caption

25 of 68

Photos:The ISIS terror threat

People search through debris after an explosion at a Shiite mosque in Qatif, Saudi Arabia, on Friday, May 22. ISIS claimed responsibility for the attack, according to tweets from ISIS supporters, which included a formal statement from ISIS detailing the operation.

Hide Caption

26 of 68

Photos:The ISIS terror threat

Iraqi soldiers fire their weapons toward ISIS group positions in the Garma district, west of the Iraqi capital of Baghdad, on Sunday, April 26. Pro-government forces said they had recently made advances on areas held by Islamist jihadists.

Hide Caption

27 of 68

Photos:The ISIS terror threat

A member of Afghanistan's security forces stands at the site where a suicide bomber on a motorbike blew himself up in front of the Kabul Bank in Jalalabad, Afghanistan, on Saturday, April 18. ISIS claimed responsibility for the attack. The explosion killed at least 33 people and injured more than 100 others, a public health spokesman said.

Hide Caption

28 of 68

Photos:The ISIS terror threat

Iraqi counterterrorism forces patrol in Ramadi on April 18.

Hide Caption

29 of 68

Photos:The ISIS terror threat

Thousands of Iraqis cross a bridge over the Euphrates River to Baghdad as they flee Ramadi on Friday, April 17.

Hide Caption

30 of 68

Photos:The ISIS terror threat

Yazidis embrace after being released by ISIS south of Kirkuk, Iraq, on Wednesday, April 8. ISIS released more than 200 Yazidis, a minority group whose members were killed, captured and displaced when the Islamist terror organization overtook their towns in northern Iraq last summer, officials said.

Hide Caption

31 of 68

Photos:The ISIS terror threat

Kurdish Peshmerga forces help Yazidis as they arrive at a medical center in Altun Kupri, Iraq, on April 8.

Hide Caption

32 of 68

Photos:The ISIS terror threat

A Yazidi woman mourns for the death of her husband and children by ISIS after being released south of Kirkuk on April 8. ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts.

Hide Caption

33 of 68

Photos:The ISIS terror threat

People in Tikrit inspect what used to be a palace of former President Saddam Hussein on April 3.

Hide Caption

34 of 68

Photos:The ISIS terror threat

On April 1, Shiite militiamen celebrate the retaking of Tikrit, which had been under ISIS control since June. The push into Tikrit came days after U.S.-led airstrikes targeted ISIS bases around the city.

Hide Caption

35 of 68

Photos:The ISIS terror threat

Iraqi security forces launch a rocket against ISIS positions in Tikrit on Monday, March 30.

Hide Caption

36 of 68

Photos:The ISIS terror threat

The parents of 19-year-old Mohammed Musallam react at the family's home in the East Jerusalem Jewish settlement of Neve Yaakov on Tuesday, March 10. ISIS released a video purportedly showing a young boy executing Musallam, an Israeli citizen of Palestinian descent who ISIS claimed infiltrated the group in Syria to spy for the Jewish state. Musallam's family told CNN that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.

Hide Caption

37 of 68

Photos:The ISIS terror threat

Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.

Hide Caption

38 of 68

Photos:The ISIS terror threat

Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants abducted at least 220 Assyrians in Syria.

Hide Caption

39 of 68

Photos:The ISIS terror threat

Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on Wednesday, February 4. Al-Kasasbeh's son, Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was recently released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.

Hide Caption

40 of 68

Photos:The ISIS terror threat

A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.

Hide Caption

41 of 68

Photos:The ISIS terror threat

Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.

Hide Caption

42 of 68

Photos:The ISIS terror threat

Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.

Hide Caption

43 of 68

Photos:The ISIS terror threat

Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.

Hide Caption

44 of 68

Photos:The ISIS terror threat

ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.

Hide Caption

45 of 68

Photos:The ISIS terror threat

An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.

Hide Caption

46 of 68

Photos:The ISIS terror threat

Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.

Hide Caption

47 of 68

Photos:The ISIS terror threat

Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.

Hide Caption

48 of 68

Photos:The ISIS terror threat

A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.

Hide Caption

49 of 68

Photos:The ISIS terror threat

Iraqi special forces search a house in Jurf al-Sakhar, Iraq, on Thursday, October 30, after retaking the area from ISIS.

Hide Caption

50 of 68

Photos:The ISIS terror threat

ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.

Hide Caption

51 of 68

Photos:The ISIS terror threat

Kurdish fighters walk to positions as they combat ISIS forces in Kobani on Sunday, October 19.

Hide Caption

52 of 68

Photos:The ISIS terror threat

Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on October 18.

Hide Caption

53 of 68

Photos:The ISIS terror threat

Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.

Hide Caption

54 of 68

Photos:The ISIS terror threat

Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.

Hide Caption

55 of 68

Photos:The ISIS terror threat

Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.

Hide Caption

56 of 68

Photos:The ISIS terror threat

A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.

Hide Caption

57 of 68

Photos:The ISIS terror threat

Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.

Hide Caption

58 of 68

Photos:The ISIS terror threat

A elderly man is carried after crossing the Syria-Turkey border near Suruc on Saturday, September 20.

Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.

Hide Caption

61 of 68

Photos:The ISIS terror threat

Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.

Hide Caption

62 of 68

Photos:The ISIS terror threat

Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

Hide Caption

63 of 68

Photos:The ISIS terror threat

Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.

Hide Caption

64 of 68

Photos:The ISIS terror threat

Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.

Hide Caption

65 of 68

Photos:The ISIS terror threat

A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.

Hide Caption

66 of 68

Photos:The ISIS terror threat

A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.

Hide Caption

67 of 68

Photos:The ISIS terror threat

Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.

Hide Caption

68 of 68

1.ISIS could unleash a crash program to attack the West

The nightmare scenario is that ISIS leaders initiate a crash program to launch attacks in the West. They are well-positioned to unleash such carnage if they choose.

ISIS, which calls itself the Islamic State but formerly was known as the Islamic State in Iraq and Syria, is thought to have tens of millions of dollars in cash reserves and it operates training camps on a scale last seen in Taliban-run Afghanistan. The 9/11 operation, by point of reference, cost $500,000.

European officials tell CNN up to 1,000 European extremists are believed to have joined ISIS. Dozens of Americans are probably fighting with the group, too. This gives ISIS the opportunity to train them and send them back home to launch attacks.

These European fighters also could pose a threat to the United States because many Europeans do not need a visa to enter the U.S.

There also are fears these Western fighters might be equipped with sophisticated devices that could evade airport security. Early this year, U.S. officials became worried that some AQAP bomb-makers trained by master-bombmaker Ibrahim al Asiri may have traveled to Syria.

This combination of threats was described by U.S. Attorney General Eric Holder as "more frightening than anything I think I've seen as attorney general," in an interview with ABC News several weeks ago.

So far, though, ISIS has not seen attacking the West as anything near a priority.

Its focus instead has been on expanding the territory it controls in Syria and Iraq.

The political turmoil brought about by the Arab Spring has made the ultimate dream of global jihadists -- the adoption of their kind of Islamic rule across the Arab world -- seem tantalizingly close. Attacking the West, which for al Qaeda leaders was always a means to this end, has become something of a sideshow.

It is possible that the limited aerial campaign initiated by U.S. President Barack Obama will not see a significant switch of focus from ISIS towards attacking the West. The White House's stated objectives do not include degrading the group.

ISIS -- previously known as al Qaeda in Iraq -- has never prioritized targeting Western soil, instead preferring to focus on fighting "infidels" at home.

In the decade since the Jordanian terrorist Abu Musab al-Zarqawi created the group, it has not been directly behind any plot on Western soil, even during the years the United States military engaged it in intense counter-insurgency operations across Iraq.

By contrast, in the decade after the September 11 terrorist attacks, al Qaeda operatives in Pakistani tribal areas provided wave after wave of Western recruits training on how to make bombs out of chemicals and components readily available in home-improvement and beauty-supply stores in the West.

To date, only one suspected ISIS recruit who has returned to Europe is alleged to have built such a device.

In February, French police arrested a man they identified as Ibrahim B., a 23-year-old French-Algerian, and retrieved three soda cans filled with nearly a kilogram (about two pounds) of the high explosive TATP from his Cannes apartment. French police suspect that in the 18 months he fought in Syria, he learned how to make TATP, an unstable and difficult-to-transport high explosive used to build detonators in multiple al Qaeda plots against the West.

It is not clear whether ISIS signed off on his alleged plot.

While some Western recruits are taught how to make improvised explosive devices in Syria, there is little indication yet that the group has created a training program tailored to attacking the West. The worry is that that could change. After a decade of insurgency in Iraq, no other group has more expertise in making IEDs.

JUST WATCHED

Iraq conflict creating strange alliances

MUST WATCH

JUST WATCHED

Iraqi family counts cost of ISIS conflict

MUST WATCH

Iraqi family counts cost of ISIS conflict02:22

Photos:Photos: Syrian civil war in 2014

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A wounded child walks at a makeshift hospital in the rebel-held town of Douma after being injured in a reported airstrike by government forces on Tuesday, December 23. Douma, located near Damascus, has been under government siege for more than a year, with residents facing dwindling food and medical supplies.The United Nations estimates nearly 200,000 people have been killed in Syria since an uprising in March 2011 spiraled into civil war.

Hide Caption

1 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A Christmas tree and a crèche made out of rubble are set up on a square in the, predominantly Christian, government-held Hamidiyeh neighborhood of Homs on Monday, December 22.

Hide Caption

2 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrian children await medical treatment at a makeshift clinic in the besieged rebel town of Douma, on Sunday, December 21, near Damascus.

Hide Caption

3 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A Syrian rebel fighter keeps an eye on government troops in Aleppo, Syria, on Wednesday, December 17.

Hide Caption

4 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man drives his motorcycle through a puddle in Aleppo on Wednesday, November 26.

Hide Caption

5 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A Syrian opposition fighter fires at Bashar al-Assad Regime forces in the Handarat district of Aleppo on Thursday, November 20.

Hide Caption

6 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrian boys play in the ruins of a destroyed building in Aleppo on Tuesday, November 18.

Hide Caption

7 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A wounded man is treated at a makeshift hospital in Damascus, Syria, following a reported air strike by government forces on Tuesday, November 11.

Hide Caption

8 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Members of the Syrian Civil Defense carry an injured man after an alleged air strike in Aleppo on November 11.

Hide Caption

9 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A member of the Syrian Civil Defense walks through a cloud of dust after an alleged air strike by government forces in Aleppo on November 11.

Hide Caption

10 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A father cries over his son at a physical therapy center in Eastern al-Ghouta outside Damascus on Thursday, November 6. The boy had his leg tendons cut after he was injured in an airstrike four months before.

Hide Caption

11 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A blindfolded man suspected of passing military information to the Syrian government waits to be interrogated by Free Syrian Army fighters Monday, October 6, in Aleppo.

Hide Caption

12 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Medics at a field hospital in Douma, Syria, attend to a man who was injured in what activists said were two airstrikes carried out by forces loyal to Syrian President Bashar al-Assad on Saturday, September 20.

Hide Caption

13 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Free Syrian Army fighters rest inside a damaged room in Aleppo on Tuesday, September 16, during what activists said were clashes with forces loyal to al-Assad.

Hide Caption

14 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrian government forces walk down a street in Halfaya, Syria, after taking the city from rebel forces on Friday, September 12.

Hide Caption

15 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Al-Qaeda-linked rebels from Syria gather around vehicles carrying U.N. peacekeepers from Fiji before releasing them Thursday, September 11, in the Golan Heights. The 45 peacekeepers were captured in the Golan Heights after rebels seized control of a border crossing between Syria and the Israeli-occupied territory.

Hide Caption

16 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrian opposition fighters take position behind sandbags in Aleppo on Thursday, September 11.

Hide Caption

17 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrians fleeing the violence stand next to their belongings as they attempt to cross into Turkey on Sunday, September 7.

Hide Caption

18 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A boy looks at bodies lying outside a hospital after a barrel-bomb attack in Aleppo on Friday, September 5.

Hide Caption

19 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A U.N. convoy moves in the buffer zone near the Golan Heights as they are escorted by Syrian rebel fighters near the Syrian village of Jubata Al Khashab on Tuesday, September 2.

Hide Caption

20 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Residents of Aleppo remove a body from debris on Friday, August 29, after what activists claim was shelling by forces loyal to al-Assad.

Hide Caption

21 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Druze men watch from the Golan Heights side of the Quneitra border with Syria as smoke rises during fighting between rebels and forces loyal to al-Assad on Wednesday, August 27.

Hide Caption

22 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – This image was taken during a government guided tour in Mleiha, Syria, one day after Syrian government forces retook the town after a months-long battle with rebels, according to a military source and state television on Friday, August 15.

Hide Caption

23 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Residents inspect the rubble of destroyed buildings in Aleppo after Syrian regime helicopters allegedly dropped barrel bombs there on Wednesday, August 13.

Hide Caption

24 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Smoke trails over Aleppo following barrel bombs that were allegedly dropped by the Syrian regime on an opposition-controlled area on Monday, August 11.

Hide Caption

25 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Photographs of victims of the Bashar al-Assad regime are displayed as a Syrian Army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The briefing on Thursday, July 31, was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar was apparently a witness to al-Assad's brutality and has smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents.

Hide Caption

26 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrian rebel fighters take up positions behind sandbags in Aleppo on Wednesday, July 30.

Hide Caption

27 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – People carry an injured man away from the site of an airstrike, reportedly carried out by Syrian government forces, in Aleppo on Sunday, July 27.

Hide Caption

28 of 85

Photos:Photos: Syrian civil war in 2014

Hide Caption

29 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Rebel fighters execute two men Friday, July 25, in Binnish, Syria. The men reportedly were charged by an Islamic religious court with detonating several car bombs.

Hide Caption

30 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A rebel fighter stands on a dust-covered street in Aleppo on Monday, July 21.

Hide Caption

31 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man clears debris at the site of an alleged barrel-bomb attack in Aleppo on Tuesday, July 15.

Hide Caption

32 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A woman walks amid debris after an airstrike by government forces July 15 in Aleppo.

Hide Caption

33 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – People walk on a dust-filled street after a reported barrel-bomb attack in Aleppo on Monday, July 7.

Syrian civil war in 2014 – A man carries a girl injured in a reported barrel-bomb attack by government forces June 3 in Aleppo.

Hide Caption

36 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A rebel fighter loads an anti-tank cannon outside Latakia, Syria, on Sunday, June 1.

Hide Caption

37 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A rescue worker pulls a girl from rubble in Aleppo on June 1 after reported bombing by government forces.

Hide Caption

38 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A giant poster of Syrian President Bashar al-Assad is seen in Damascus, Syria, on Saturday, May 31, as the capital prepares for presidential elections.

Hide Caption

39 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Portraits of al-Assad dominate the cityscape in central Damascus on Tuesday, May 27. Al-Assad is firmly in power three years into the civil war, while the opposition remains weak and fragmented and extremists grow in numbers and influence.

Hide Caption

40 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – The father of a 3-month-old girl weeps Monday, May 26, after she was pulled from rubble following a barrel-bomb strike in Aleppo.

Hide Caption

41 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A woman stands in a heavily damaged building in Aleppo on May 26.

Hide Caption

42 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – An injured man lies in a hospital bed after alleged airstrikes by government forces in Aleppo on Sunday, May 18.

Hide Caption

43 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Buildings in Homs, Syria, lie in ruins Saturday, May 10, days after an evacuation truce went into effect. Thousands of displaced residents returned to the city.

Hide Caption

44 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Rescuers carry a man wounded by a mine in the Bustan al-Diwan neighborhood of Homs on May 10.

Hide Caption

45 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A Syrian woman carries a suitcase along a street in the Juret al-Shayah district of Homs on May 10.

Hide Caption

46 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Residents carry their belongings in the al-Hamidieh neighborhood of Homs on May 10.

Hide Caption

47 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A woman injured when a mine went off is carried in Homs on May 10.

Hide Caption

48 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Residents return to damaged dwellings in Homs on May 10.

Hide Caption

49 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Debris lies on a deserted street in Homs on Thursday, May 8.

Hide Caption

50 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A mosque is seen through shattered glass in Homs, where an evacuation truce went into effect on Wednesday, May 7.

Hide Caption

51 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A wounded man is treated at a makeshift hospital in Aleppo on Sunday, May 4.

Hide Caption

52 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Debris rises in what Free Syrian Army fighters said was an operation to strike a checkpoint and remove government forces in Maarat al-Numan, Syria, on Monday, May 5.

Hide Caption

53 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man helps a woman through debris after reported airstrikes by government forces on Thursday, May 1, in the Halak neighborhood of Aleppo.

Hide Caption

54 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrians gather at the site of reported airstrikes in Aleppo on May 1. According to the Syrian Observatory for Human Rights, at least 33 civilians were killed in the attack.

Hide Caption

55 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A woman runs after two barrel bombs were thrown, reportedly by forces loyal to Syrian President Bashar al-Assad in Aleppo on May 1.

Hide Caption

56 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A boy runs in Aleppo on Sunday, April 27, after what activists said were explosive barrels thrown by forces loyal to al-Assad.

Hide Caption

57 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Security and emergency medical personnel work at the site of a car bomb explosion Monday, April 14, in the Ekremah neighborhood of Homs.

Hide Caption

58 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – In this photo released by the state-run SANA news agency, Syrian forces take positions during clashes with rebels near the town of Rankous, Syria, on Sunday, April 13.

Hide Caption

59 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Flames engulf a vehicle following a car bomb Wednesday, April 9, in the Karm al-Loz neighborhood of Homs.

Hide Caption

60 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man carries a child who was found in the rubble of an Aleppo building after it was reportedly bombed by government forces on Monday, March 18.

Hide Caption

61 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – An elderly man and a child walk among debris in a residential block of Aleppo on March 18.

Hide Caption

62 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A woman with blood on her face carries a child following a reported airstrike by government forces Saturday, March 15, in Aleppo.

Hide Caption

63 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – People attempt to comfort a man in Aleppo after a reported airstrike by government forces on Sunday, March 9.

Hide Caption

64 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Buildings in Homs lay in ruins on March 9.

Hide Caption

65 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrian forces fire a cannon and a heavy machine gun loaded on a truck as they fight rebels in the Syrian town of Zara on Saturday, March 8.

Hide Caption

66 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A handout photo released by SANA shows Syrian President Bashar al-Assad speaking March 8 during a meeting in Damascus to mark the 51st anniversary of the 1963 revolution, when Baath Party supporters in the Syrian army seized power. Al-Assad said the country will go on with reconciliation efforts along with its fight against terrorism.

Hide Caption

67 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrians inspect the rubble of destroyed buildings in Aleppo following a reported airstrike by Syrian government forces on Friday, March 7.

Hide Caption

68 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – People dig through the rubble of a building in Damascus that was allegedly hit by government airstrikes on Thursday, February 27.

Hide Caption

69 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A boy walks ahead of men carrying the body of his mother in Aleppo on Saturday, February 22. According to activists, the woman was killed when explosive barrels were thrown by forces loyal to al-Assad.

Hide Caption

70 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man holds a baby who survived what activists say was an airstrike by al-Assad loyalists Friday, February 14, in Aleppo.

Hide Caption

71 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – In this photo provided by the anti-government activist group Aleppo Media Center, Syrian men help survivors out of a building in Aleppo after it was bombed, allegedly by a Syrian regime warplane on Saturday, February 8.

Hide Caption

72 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrians gather at a site hit by barrel bombs, allegedly dropped by a regime helicopter on the opposition-controlled Mesekin Hananu district of Aleppo on February 8.

Hide Caption

73 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – In this handout photo released by the state-run SANA news agency on February 8, civilians wave national flags in Damascus as they take part in a rally in support of President al-Assad.

Hide Caption

74 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man stands next to debris in the road following a reported airstrike by Syrian government forces in Aleppo on February 8.

Hide Caption

75 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Medical personnel look for survivors after a reported airstrike in Aleppo on Saturday, February 1.

Hide Caption

76 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Syrians carry a dead body following an airstrike on February 1.

Hide Caption

77 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man walks amid debris and dust on January 31.

Hide Caption

78 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – An injured man is covered in dust after an airstrike on January 29.

Hide Caption

79 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A man tries to fix electrical wires in Aleppo on January 27.

Hide Caption

80 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Rebels and civilians check out a crater that activists say resulted from a Syrian government airstrike on an Aleppo bus station on Tuesday, January 21.

Hide Caption

81 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Men rush to a site that Syrian government forces reportedly hit in Aleppo on January 21.

Hide Caption

82 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – Buildings lie in ruins in Aleppo on Sunday, January 19, after reported air raids by Syrian government planes.

Hide Caption

83 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A child collects items from a garbage pile in Douma, northeast of the capital, on Saturday, January 18.

Hide Caption

84 of 85

Photos:Photos: Syrian civil war in 2014

Syrian civil war in 2014 – A piece of exploded mortar lies in a street in Daraya, a Syrian city southwest of Damascus, on Friday, January 17.

Hide Caption

85 of 85

The most immediate threat to the West comes from hundreds of extremists who have returned home after fighting with terrorist groups in Syria. While little evidence has emerged so far that ISIS has directed them to launch attacks, their urban warfare skills would make them especially dangerous.

The first terrorist attack on Western soil linked to Syria probably followed this trajectory. Mehdi Nemmouche, a French-Algerian who spent a year in Syria and was recruited into ISIS, has been charged with gunning down four people at a Jewish museum in Brussels, Belgium, in May. ISIS itself did not claim responsibility for the attack, suggesting to investigators that Nemmouche planned the attack himself.

Among Western countries Europe has by far the greatest number of returnees.

"The threat of attacks has never been greater -- not at the time of 9/11, not after the war in Iraq -- never," a European counterterrorism official told CNN in June. He envisaged a flood of small-scale but effective and chilling attacks similar to the Brussels shooting.

European counterterrorism officials are worried the gains made by ISIS in Iraq will lead to a surge of travel to the region. In identifying who has traveled, they are often playing catchup.

"In most cases, we know within two weeks a guy has gone to Syria. But 10%-15% of the time, it can be several months before we figure it out. Inevitably, there will be some we have no idea about," one official told CNN.

But even those they know about are difficult to track. Nemmouche was on a watch list when he returned to Europe. European officials tell CNN it is impossible, because of the cost, to conduct 24-hour surveillance on any but a small fraction of people who have returned from Syria.

3.'Lone wolves' could lash out

The Boston Marathon bombings illustrated the danger posed by extremists learning bomb-making skills over the Internet without having to travel to jihadist encampments overseas.

European officials say anger about events in Syria and Iraq and excitement about the gains made by ISIS have spiked radicalization to unprecedented levels across the continent.

Though the animus is not directed as squarely against the West as it was during the Iraq war, ISIS' viscerally anti-Western ideology is attracting a growing following in extremist circles in Europe. It matters little that European countries are not currently involved in air strikes: ISIS supporters believe all Western countries are working together to attack Islam.

The Costs of inaction

The likely increased terrorist threat in the West that will arise in the coming weeks and months from U.S. air strikes in Iraq should be weighed against the risks of letting ISIS grow unchecked.

If ISIS is able to consolidate its territorial gains, it could set up training camps to rival any run by al Qaeda in Afghanistan before 9/11. Though its focus may be in Iraq and Syria for now, its viscerally anti-Western ideology means it would be foolish to discount it as a major potential future threat.

Though the unfolding humanitarian crisis in Iraq may have left the Obama administration little choice but to act, the concern is the limited US aerial campaign has now stirred the hornet's nest without doing anything to decisively degrade ISIS.

And there could be one other unintended consequence. A rally-round-the-flag effect may now strengthen the ISIS position in the global jihadist movement, leading to even higher levels of fundraising and recruitment.

In recent months the group has faced fierce criticism from al Qaeda and others for its declaration of an Islamic caliphate and its brutal methods. But with the budding Islamic State now being bombarded by the "Crusaders," such criticisms may fade, at least for a time.